The subject invention relates generally to thermal ink jet printers, and is directed more particularly to a technique for maintaining consistently high print quality in the event of unplanned or unforseen delays in printing a particular document or page.
Thermal ink jet printers utilize thermal ink jet printheads that comprise an array of precision formed nozzles, each of which is in communication with an associated ink containing chamber that receives ink from a reservoir. Each chamber includes an ink drop firing resistor which is located opposite the nozzle so that ink can collect between the ink drop firing resistor and the nozzle. The ink drop firing resistor is selectively heated by voltage pulses to drive ink drops through the associated nozzle opening in the orifice plate. During each pulse, the ink drop firing resistor is rapidly heated, which causes the ink directly adjacent the ink drop firing resistor to vaporize and form a bubble. As the vapor bubble grows, momentum is transferred to the ink between the bubble and the nozzle, which causes such ink to be propelled through the nozzle and onto the print media.
A consideration with the operation of thermal ink jet printheads is the variation in print density that results from the printhead cooling that takes place during delays that occur while printing a particular output. Such variation in print density obtains because the physical properties of the ink (most notably the viscosity) are temperature-dependent. Volume of the ejected drop and spot size on the media depend on the physical properties of the ink, and hence on the ink temperature. Finally, the ink temperature and the printhead temperature are very nearly the same; so the printhead temperature determines the ink temperature, which determines the ink properties, which determine the image density on the media.
If the printing of a particular output such as a graphics image is not accomplished generally continuously, for example, wherein the printer has to repeatedly wait until further data is received, print density shifts occur, which generally look like bands of different print densities across the printed output. The occurrence of such print density shifts is sometimes called "wait time banding."
The problem of wait time banding has been addressed by suggesting that applications software should be faster to reduce wait times. While such approach might alleviate wait time banding to some degree, it requires various parties to address the problem, and moreover would probably not address the development of higher speed thermal ink jet printers with which the wait time banding problem would be more aggravated.